The invention concerns a dry-lubricated bearing with a sliding layer containing carbon between the bearing surfaces.
Bearings serve as support (guidance) of the moving parts of a machine inside stationary parts. Such bearings are used, for example, in rotary X-ray anodes.
In order to reduce the frictional resistance between bearing surfaces, lubricants are applied which reduce the wear and tear on the material and also the energy consumption.
In the friction between bearing surfaces, the frictional or tangential force K is dependent on the coefficient of friction .mu. between the bearing surfaces and on the load P with which the surfaces rub on or against one another: K=.mu..multidot.P.
A bearing surface means the surface of a component which bears against the surface of another component when there is a certain contact force exerted on one component by the other component.
The coefficient of friction .mu. is dependent on the surface roughness of the bearing surfaces, but in the case of smooth surfaces on the pairing of materials, i.e. on the materials of the opposed bearing surfaces. It is known how to change the coefficient of friction of dry rubbing by covering the bearing surfaces with other kinds of substances in order to reduce the friction and to reduce the wear of the bearing surfaces.
On account of their material properties, the known dry lubricants can be split up into different groups:
1. substances which have good sliding friction properties because of a plate-like crystalline structure, such as graphite or even the chalcogenides of metals like molybdenum, tungsten and niobium; PA1 2. malleable metals such as gold, silver, lead or tin; PA1 3. hard substances such as silicides, borides, nitrides or carbides.
The substances mentioned can be placed loosely between the bearing surfaces or deposited as thin layers bonded to the bearing surface by means of a conventional thin film technology process, such as vapour deposition or cathode sputtering.
All these dry lubricants have their particular advantages and disadvantages.
A disadvantage peculiar to graphite as a dry lubricant is that while in normal atmospheric conditions it has a low sliding friction coefficient (.mu..congruent.0.1-0.2), which is desirable, in a dry atmosphere it displays a much greater sliding friction coefficient (.mu..congruent.0.8); the result of this is that there is considerable wear of the bearing surfaces on a dry atmosphere, since the graphite is very soft.
Friction problems at not very high temperatures can be solved effectively by using chalcogenides of the type mentioned above, but the use of these materials becomes problematical at high temperatures. MoS.sub.2, for example, decomposes in air at temperatures above 400.degree. C. This lack of thermal stability therefore limits the employment of materials which would otherwise be suitable as dry lubricants. In contrast to graphite, MoS.sub.2 has a very low sliding friction coefficient (.mu.=0.04) in a dry atmosphere, while under normal atmospheric conditions the sliding friction coefficient .mu. rises to .about.0.2.
A general disadvantage of materials with a plate-like structure--and this applies both to graphite and to MoS.sub.2 --is their insufficient abrasion resistance and relative softness.
Malleable metals such as gold, silver, lead and tin are good dry lubricants as these materials have a comparatively low sliding friction coefficient (.mu..congruent.0.2-0.4). Since these metals are extremely soft, however,--which is what on the other hand gives them their good sliding properties--they also only have a low abrasion resistance.
In order to provide lubricating layers having a higher abrasion resistance, harder materials such as silicides, borides, nitrides and carbides have been used on bearing surfaces.
It has been shown that it is possible to achieve a good resistance to abrasion with these harder materials, but the sliding friction coefficients in these cases are, with values of .mu..congruent.0.3 to 0.7, far above the values of the materials which are suitable as dry lubricants because of their low sliding friction coefficients, graphite (.mu..congruent.0.1-0.2) or molybdenum sulfide MoS.sub.2 (.mu..congruent.0.04).